Nucleic acids are the DNA and RNA that govern the genetic information of living creatures. On the other hand, a peptide nucleic acid (PNA) is a modified nucleic acid in which the sugar-phosphate skeleton of a nucleic acid has been converted into an N-(2-aminoethyl)glycine skeleton (FIG. 1). The sugar-phosphate skeletons of DNA/RNA are negatively charged under neutral conditions and exhibit electrostatic repulsion between the complementary strands, but since the backbone structure of PNA itself has no charge, there is no electrostatic repulsion. PNA therefore has a high duplex-forming ability and a high base sequence recognition ability in comparison with conventional nucleic acids. Furthermore, since PNA is very stable against in vivo nuclease/protease and is not decomposed thereby, its application in gene therapy as an antisense molecule has been investigated.
Modifying conventional techniques that employ DNA as a medium so that they can be used with PNA can compensate for the defects of DNA that could not be overcome previously. For example, it is possible to apply PNA to the “DNA microarray technology” that carries out a systematic analysis of a large amount of genetic information at high speed, and to the “molecular beacon” that has been developed recently as a probe that can detect by fluorescence a specifically recognised base sequence. Since these techniques use DNA as a medium, which has poor enzyme resistance, when employing these techniques it is necessary to carry out precise sampling. Satisfying this requirement is the key to enhancing the above-mentioned techniques.
On the other hand, since PNA is completely resistant to enzymes, the use of PNA as a replacement for DNA in the DNA microarray technology and the molecular beacon is anticipated to eliminate the defects of the above-mentioned techniques and to derive further advantages.
There are a large number of fields, in addition to the DNA microarray technology and the molecular beacon, that are anticipated to advance as a result of the use of PNA, and in these fields it is necessary to efficiently functionalise PNA, that is to say, to design a novel PNA monomer by the efficient introduction of a functional molecule to a PNA monomer.
Since methods for synthesising a PNA oligomer employ the commonly used solid phase peptide synthesis, PNA monomer units can be classified into two types in terms of the PNA backbone structure, that is to say, the Fmoc type PNA monomer unit and the tBoc type PNA monomer unit (FIG. 2).
A method for synthesising the Fmoc type PNA monomer unit has already been established, its oligomers can be synthesised by means of a standard automatic DNA synthesiser, and it can be synthesised on a small scale by the route below:
(X denotes guanine, thymine, cytosine or adenine.)
The first PNA employed the tBoc type PNA monomer unit as described below:
after which a more efficient synthetic method:
was established.
However, as described above, since the Fmoc type, which is easy to handle, has been developed, the use of the tBoc type is becoming less frequent.
However, when introducing a functional molecule other than the four nucleic acids guanine, thymine, cytosine and adenine, for example, when introducing a photofunctional molecule, since the functional molecule that is to be introduced is often unstable under alkaline conditions, the tBoc type PNA backbone structure, which does not employ alkaline conditions, is very useful. With regard to a “process for producing t-butoxycarbonylaminoethylamines and amino acid derivatives”, there is already a patent application filed by the present inventors as Japanese Patent Application No. 2000-268638.
Other than the above process, 5 examples of the synthesis of a monomer unit for a photofunctional PNA oligomer are known. All these cases employ the above-mentioned route, but their yields are either not described or very low (Peter E. Nielsen, Gerald Haaiman, Anne B. Eldrup PCT Int. Appl. (1998) WO 985295 A1 19981126, T. A. Tran, R.-H. Mattern, B. A. Morgan (1999) J. Pept. Res, 53, 134-145, Jesper Lohse et al. (1997) Bioconjugate Chem., 8, 503-509, Hans-georg Batz, Henrik Frydenlund Hansen, et al. PCT Int. Appl. (1998) WO 9837232 A2 19980827, Bruce Armitage, Troels Koch, et al. (1998) Nucleic Acid Res., 26, 715-720, Hans-georg Batz, Henrik Frydenlund Hansen, et al.) Furthermore, since the structures of the compounds used have the characteristic of being comparatively stable under alkaline conditions, it is surmised that, when a chromophore that is unstable under alkaline conditions is present, an efficient synthesis by a method similar to the above-mentioned conventional method, that is to say, route A below, is not possible.

There is therefore a strong desire for the establishment of a technique that functionalises a PNA monomer efficiently as well as for the development of a functional PNA monomer such as, for example, a photofunctional PNA monomer.